Fe-Zn manganite spinels and their carbonate precursors: preparation, characterization and catalytic activity

Iron-zinc manganites at different Fe content [x = Fe/(Fe + Zn) = 0, 0.01, 0.05, 0.10] were prepared by thermal decomposition of carbonate precursors obtained by coprecipitation at constant pH. Precursors were characterized by diffuse reflectance spectroscopy (DRS), thermogravimetric analysis (TGA-DTA) and magnetic susceptibility measurements. All samples are made by a single rhodochrosite-like phase, FexZnyMn(1−x−y)CO3, in which besides Zn2+ and Mn2+, only 5% of the total iron was Fe2+, the remaining 95% surprisingly being Fe3+ ions. The carbonate monophasic precursors were decomposed at 723 and 973 K in air and solid solutions of iron in the zinc manganite spinel-like phase were obtained at both temperatures. Zinc manganite, ZnMn2O4, is a 'normal' spinel in which the Zn2+ ions occupy the tetrahedral sites and Mn3+ ions the octahedral sites of the crystal lattice. When iron is loaded, it is present as Fe3+ ions in the octahedral sites of the spinel together with Mn3+ ions. The catalyst reducibility in H2 was studied by temperature-programmed reduction (TPR). The iron-containing catalysts resulted to be slightly more resistant towards reduction than pure ZnMn2O4, suggesting that Fe3+ ions play a stabilizing effect on the spinel structure. Results of a Mössbauer investigation on the iron chemical state and its coordination symmetry are extensively reported. Some preliminary results of the catalytic behaviour for the reduction of NO by hydrocarbons are presented.